The modifications of membrane components, mainly proteins, that occur in cells transformed by a temperature-sensitive mutant of Rous sarcoma virus are studied in order to ascertain the role of such modifications in alterations of cell properties that are expressed at or mediated by the cell surface. Cell membranes are isolated and their composition determined by electrophoretic and chromatographic procedures. Membrane surface properties are investigated by the use of lectins and labeling with surface-selective techniques.